The field of the invention is magnetic resonance imaging (MRI) and systems. More particularly, the invention relates to a system including and method for using an improved MRI coil design that can be used to improve radio frequency (RF) detection and/or B0 field shimming.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Spatial variation in the static background field (B0) commonly occurs in areas about the head and body, where tissue magnetic susceptibility changes abruptly, which distorts the B0 field lines and creates off-resonance signals. Perturbation of the B0 field can cause image artifacts, such as geometric distortion and signal loss due to intravoxel dephasing, particularly at high field strengths, such as 3 Tesla and 7 Tesla. This field inhomogeneity remains an obstacle to applications such as functional MRI (fMRI) of the brain with echo planar imaging (EPI) sequences, which rely on long echo trains that are sensitive to off-resonance effects. Artifacts are particularly pronounced in brain areas near the sinus cavities, which suffer from strong B0 inhomogeneity caused by the air-filled sinus cavities in close proximity to the tissue being imaged. A common way to partially overcome this obstacle is to reduce the echo train length with parallel imaging methods, such as GRAPPA, but this comes at the expense of signal-to-noise ratio by reducing the accuracy of the fMRI measurement.
In conventional MRI scanners, inhomogeneity is compensated for using body-sized shim coils inside the bore to generate first and second-order spherical harmonic fields. Unfortunately, these solutions are ill suited to match (and cancel) the rapid variation of the B0 field in the head, particularly in the sinus regions, because such solutions generate fields that are spatially slowly-varying. Recently, arrays of multiple loop coils placed in the vicinity of the head have been used to generate higher-order field shapes, providing more accurate compensation of B0 inhomogeneity in mice (Juchem C, Brown P B, Nixon T W, McIntyre S, Rothman D L, de Graaf R A. Multicoil shimming of the mouse brain. Magn Reson Med. 66(3); 2011:893-900.) and humans (Juchem C, Nixon T W, McIntyre S, Boer V O, Rothman D L, de Graaf R A. Dynamic multi-coil shimming of the human brain at 7 T. J Magn Reson. 212(2); 2011:280-8.). As described by Juchem, et al., field maps of each loop are obtained and each element is driven with the necessary amount of direct current to shim out the B0 inhomogeneity in the sample. In this approach, RF transmit and/or receive coils are nested inside an array of 32 or more DC shim coils. Unfortunately, placing shim coils inside the RF coils leads to excessive shielding and interaction with the shim coils and loss of RF transmit field and/or receive sensitivity. Also, the presence of separate shim loops consumes space and limits the number of RF receive coils that can be placed close to the body, in addition to reducing the SNR of these coils.
Therefore, it would be desirable to have a system and method to shim or compensate for B0 inhomogeneities, such as are common about the head and elsewhere, that does not limit the pulse sequences available for imaging or undesirably shield the RF fields employed and result in loss of RF detection sensitivity or transmit efficiency.